Low bandgap conjugated polymers based on thiadiazoloquinoxaline for high performance shortwave infrared photodetection

The detection of light in the second near-infrared (NIR-II) region of the electromagnetic spectrum is critical for various applications such as bioimaging, environmental sensing and health monitoring. However, the narrow band gaps of organic semiconductors required for NIR-II photodetection increase the probability of charge injection under bias voltages and bulk thermal charge generation in the active layer, leading to a large dark current and low external quantum efficiency which results in poor detectivity. In this study, we introduce a series of low band-gap donor–acceptor type alternating conjugated polymers using thiadiazoloquinoxaline (TQ) as electron-withdrawing units and indacenodithiophene (P1 and P2) or propylenedioxythiophene as electron-donating units (P3–P5) for NIR-II photodetection. Polymers P3–P5 have much lower band gaps than P1 and P2 due to the stronger intrachain D–A interaction in the former. P3 and P5 have excellent solubility in a variety of organic solvents even at room temperature, which greatly facilitates the device fabrication process. The photodiode device based on P5 exhibited the highest specific detectivity of 2.0 × 10¹⁰ Jones at 1200 nm under −1 V bias owing to the significantly low dark current.